VII. 3-Band Cryo Data Release

5. Analysis of 3-Band Cryo Data Products

b. Photometric Performance

i. Photometric Accuracy

There is no formal requirement for photometric accuracy in the 3-Band Cryogenic data. The following analysis assesses the accuracy without concern to the Allsky photometric accuracy requirement.

We will be assessing the photometric accuracy of the WPRO (profile-fitting) photometry of bright stars using the internal repeatability of WPRO measurements. However, in order to draw the sample to study, we will be using the S/N as given by the standard aperture photometry. The reason is that the WPRO S/N includes a contribution due to the uncertainty in the Point Spread Function (PSF), essentially flooring all S/N values to be less than S/N=60 or so; whereas, the aperture photometry provides a reasonable estimate of source's S/N in a given band.

1. Highlighted Sample

For detailed study (Figures 1-4, presented below), we will first draw a clean sample of bright starsfrom high galactic latitudes, avoiding the inner Galactic Bulge. We then draw samples from other large regions of the sky stepping closer to the Plane of the Galaxy. The results are presented below.

Aperture photometry magnitude uncertainties: w?sigm < 0.01086 mag (S/N > 100)
Avoid saturation, artifacts, blending, etc
Avoid upper limits
Apply w?rchi2 threshold to avoid extended sources and poor WPRO photometry
WISE Pass 2 3-Band Cryo Multiframe Source Table

For example, the SQL command for W1 (band-1) extractions is as follows:

WHERE (glon > 90 and glon < 270 and glat > 65 and glat < 85 and (wx >= 36.4 and wx <= 4058.6 and wy >= 36.4 and wy <= 4058.6) and use_src = 1 and nb = 1 and w1frtr = 0 and w1rchi2 <= 2 and w1sat = 0 and cc_flags[1] not matches '[DHOP]' and w1sigmpro is not null and w1sigm < 0.01086) ' The extracted photometry for the unsaturated, bright S/N > 100 sources is presented below.

2. Internal Repeatability

The WISE photometric accuracy is assessed using the internal repeatability of WPRO measurements. For a given source found on M individual frames (typically ~12), WPRO estimates the flux for each one and the distribution statistics are part of the extracted information (per band) for the source. The reported statistics include weighted mean, the unbiased weighted sample variance (re: population variance) and the standard error of the mean. Internal repeatability is a proxy for photometric accuracy, although it is not sensitive to confusion noise. For our purposes here, confusion is not an important contributor to the flux uncertainty.

The W1 repeatability results for the bright S/N > 100 sample are shown in Figure 1. The panels show, per band, the standard deviation in the population (parameter w?sigP1), representing the RMS of WPRO measurements for one frame. The results show that the photometric accuracy for each band, as indicated by the repeatability RMS (w?sigP1), is ~2 to 3%, well within the 4-band Cryo requirement of 7% for bright S/N > 100 sources.

Figure 1: High Galactic Latitude Sky: 40 > GLAT < 60 deg


Figure 1a - WPRO internal repeatability RMS of W1. The red dashed line demarks the 4-Band Cryo (All Sky) Photometric Accuracy requirement, and the blue dashed horizontal line the achieved accuracy.	Figure 1b – WPRO internal repeatability RMS of W2.	Figure 1c – WPRO internal repeatability RMS of W3.

Figure 2: Moderate Galactic Latitude Sky: 20 < Glat < 40 deg


Figure 2a - WPRO internal repeatability RMS of W1.	Figure 2b – WPRO internal repeatability RMS of W2.	Figure 2c – WPRO internal repeatability RMS of W3.

Figure 3: Low Galactic Latitude Sky: 10 < Glat < 20 deg


Figure 3a - WPRO internal repeatability RMS of W1.	Figure 3b – WPRO internal repeatability RMS of W2.	Figure 3c – WPRO internal repeatability RMS of W3.

Figure 4: Low Galactic Latitude: 5 < Glat < 10 deg


Figure 4a - WPRO internal repeatability RMS of W1.	Figure 4b – WPRO internal repeatability RMS of W2.	Figure 4c – WPRO internal repeatability RMS of W3.

Figure 5: Low Galactic Latitude: 1 < Glat < 5 deg


Figure 5a - WPRO internal repeatability RMS of W1.	Figure 5b – WPRO internal repeatability RMS of W2.	Figure 5c – WPRO internal repeatability RMS of W3.

**Table 1** – Summary of Photometric Repeatability for Large Regions of the Sky

3. Summary

Observed WPRO 3-band Cryo W1 and W2 repeatability satisfies the relative photometric accuracy requirement of 4-band Cryo. W3 is clearly noisy, although the (strictly defined) S/N>100 photometry is still quite good.

Comparison of WISE W1 and W2 with Spitzer photometry (S/N > 100) is consistent with "accurate" WISE photometry, exceeding the Level-1 requirement.

Last update: 2012 December 10